The invention relates to a piston pump for conveying a fluid in the very low temperature range, with a pump casing comprising an inner casing pipe and fashioned as a vacuum housing, a cylinder being arranged in the inner casing pipe, a piston being displaceable in the longitudinal direction in this cylinder, one end of the cylinder being associated with a high-pressure side and the other end of the cylinder being associated with a low-pressure side of the piston pump, and a piston rod guide means adjoining the cylinder on the low-pressure side wherein a piston rod, connected at one end with the piston and at the other end with a drive mechanism located outside the pump casing, is displaceable in the longitudinal direction.
Conventional low-temperature pumps are used for pumping liquid nitrogen, oxygen, argon, carbon dioxide and hydrocarbon. Since, in these areas of application, the lowest usage temperature is at about -200.degree. C., and the heat of evaporation is relatively great, the conventional pumps need not meet any stringent requirements with respect to insulation and sealing. Additionally, these liquefied gases are produced in a relatively economical way.
However, the low-temperature pumps of the state of the art are not suited for pumping liquid hydrogen, on account of the high refrigeration losses occurring therein. Hydrogen exhibits merely 1/6 of the heat of evaporation of nitrogen; the temperature of liquid hydrogen is at -2530.degree. C., and its viscosity is very low. Besides, production of liquid hydrogen is costly. For these reasons, special requirements must be met by lowest-temperature pumps which are to be suitable for conveying fluids in the lowest-temperature range of below -250.degree. C., especially for pumping liquid hydrogen.
Although there are sporadic suggestions of pumps said to be suitable for conveying liquid hydrogen (see German Patent 3,621,727 and European Patent 0,069,324), these are so-called immersion pumps, installed from above in a vessel filled with liquid hydrogen. This arrangement may be suited for laboratory pumps of low pressures, as well as for relatively small conveying quantities. However, since only small-sized, lightweight drive mechanisms can be flanged to these pumps, the latter are not suitable for producing high-pressure hydrogen in larger conveying quantities, on account of the high driving forces and thus gearing masses required therefor. Besides, when engaged in high-pressure compression of hydrogen, the pumps must meet very much higher demands than when conveying small amounts at low pressures. This is due to the difficult physical properties, such as heat of evaporation, temperature, and viscosity of the liquid hydrogen.